This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved transfer system for use therein. In the process of electrostatographic printing, an electrostatic latent charge pattern is reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent charge pattern. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent charge pattern. Transfer, which is the act of transferring toner particles deposited on the electrostatic latent charge pattern, in image configuration, to a sheet of support material, may be employed in either electrophotography or electrography. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of the foregoing process. This printing machine incorporates the features of transfer apparatus of the present invention.
In the process of electrophotograpic printing, for example, as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, and image bearing member or photosensitive element having a photoconductive insulating layer is charged to a substantially uniform potential in order to sensitize its surface. Thereafter, the charged photoconductive surface is exposed to a light image of an original document. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity projected onto the photoconductive surface creating an electrostatic latent image thereon. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. Typical developer mixes generally comprise dyed or color thermoplastic particles, known in the art as toner particles, which are mixed with coarser carrier beads, such as ferromagnetic granules. The developer mix is selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. Frequently, some undesired carrier beads adhere to the photoconductive surface after the development of the electrostatic latent image. These adhering carrier beads are transferred to the sheet of support material contaminating and distorting the copy image. Consequently, it is highly desirable to minimize the number of carrier beads transferred to the sheet of support material.
Various prior art techniques have been developed to achieve this. For example, U.S. Pat. No. 3,287,150 issued to Lehman in 1966 describes a cascade development system wherein carrier beads as well as toner particles are deposited on the photoconductive surface. Thereafter, a magnetic member positioned closely adjacent to the photoconductive surface removes the carrier beads without disturbing the toner powder image deposited thereon. Similarly, co-pending application Ser. No. 312,554 filed in 1972 describes a cylindrical pick off roller positioning adjacent to a photoconductive surface to remove carrier beads deposited thereon during the development cycle. In addition, magnets have frequently been employed as seals for developer housings. One such example of this is described in co-pending application Ser. No. 477,946 filed in 1974. Finally, U.S. Pat. No. 3,713,736 issued to Sargis in 1973 discloses a magnetic cleaning roller covered with magnetizable particles positioned adjacent to the photoconductive surface after the transfer station. The magnetic cleaning roller removes residual toner particles from the photoconductive surface after the transfer of the toner particles to the sheet of support material.
None of the foregoing prior art references disclose pre-conditioning the carrier beads and toner particles adhering to the photoconductive surface prior to transferring the toner particles to the sheet of support material. It has been found that when the toner particles and carrier beads are pre-conditioned so as to reduce their attractive force between the photoconductive surface and subsequently subjected to a magnetic attraction, carrier beads are removed more completely than by either element alone or by both elements but with the magnetic attraction occurring prior to the pre-conditioning. In tests of one specific embodiment of this invention on an electrophotographic printing machine a significant reduction in carrier beads transferred to the support sheet was obtained. Without magnetic attraction and pre-conditioning approximately 3 times the acceptable number of carrier beads was transferred to the sheet of support material. With just pre-conditioning, this level was reduced somewhat. However, with the introduction of pre-conditioning and magnetic attraction the number of carrier beads transferred to the sheet of support material was reduced to a range of about 10 to 25 percent of the acceptable amount. Thus, it is necessary to pre-condition the toner particles and carrier beads prior to removing the carrier beads from the photoconductive surface if the carrier beads adhering to the support material are to be reduced reliably to an acceptable level.
Accordingly, it is a primary object of the present invention to improve the transfer apparatus by pre-conditioning the carrier beads and toner particles adhering to the photoconductive surface and removing the pre-conditioned carrier beads therefrom prior to transferring the toner particles to a sheet of support material.